Lumber grading in the United States is regulated under policies of the American Lumber Standards Committee (ALSC) and the various grading agencies authorized by the ALSC. Other countries have similar arrangements for regulating the grading of structural lumber materials. Whenever materials are used in construction the reliability and serviceability of the construction relate to the safety of the people and property involved in the use of the structure so it is very important that the grading and sorting process for these materials be carried out in the most accurate way possible consistent with the needs in assessing the structural properties of the materials being tested. The present invention provides improvements in this process that result in more accurate and reliable lumber grading.
The originator of this type of equipment was Keller U.S. Pat. No. 3,196,672. The apparatus invented by Keller is still in use in many lumber production plants throughout the world, and is the inspiration behind an improved apparatus by Bechtel et al U.S. Pat. No. 5,503,024. Even though the Keller apparatus is no longer manufactured it is still maintained and is in wide use some 45 years after it was first introduced. The two machines have similar basic mechanical features; both measure the modulus of elasticity of dimension lumber shapes by a mechanical means consisting of a constant-deflection force measurement of bending forces with a 48-inch bending span. For illustration purposes of this discussion we will describe how the invention relates to the Bechtel et al U.S. Pat. No. 5,503,024 apparatus only, however this method and apparatus may be adapted for use with other kinds of wood testing machines for lumber as well as for panel products.
In machine grading of lumber a series of machine grades are available with the modulus of elasticity property in approximately 5% increments. Grading thresholds must be set with a safety margin above the absolute minimum limit to account for variations in calibration of the equipment and other factors such as the statistical relationship between measured quantities in production equipment and measured quantities in laboratory quality control equipment. Any change that results in a more stable calibration has direct financial benefits by reducing the safety margin requirement.